Bistability and formation of low work function areas in the O2+H2 reaction on a Rh(111) surface

The catalytic O-2 + H-2 reaction on Rh(111) has been investigated in the 10(-6)-10(-5) mbar range using photoelectron emission microscopy (PEEM) as spatially resolving method. Depending on the pretreatment of the sample in which the sample was exposed to pO(2) = 2 X 10(-4) mbar at T = 770 K for varying times (t(OX)) different dynamic behavior was found. For t(OX) = 12 h we found simple bistable behavior with reaction fronts initiating transitions between an unreactive high oxygen coverage state and a reactive almost bare surface. For t(OX) = 36 h low work function (WF) areas developed in the area where two reaction fronts collided. For very long oxygen pretreatments with t(OX) = 48 h the formation of secondary fronts traveling backwards from the area where two primary fronts collided were seen. The properties of the low WF areas which have been tentatively assigned to subsurface oxygen were studied in adsorption and titration experiments. (C) 2000 American Institute of Physics. [S0021-9606(00)70635-3].